Many types of communications between remote locations rely on the use of the public switched telephone networks (PSTN). The most typical use of the telephone network is the dial-up of a directory number to establish a talking path between an originating station and a destination station, whereupon the telephone line remains busy for as long as the audio or data signals are being transmitted. After the communication is terminated, both stations go on-hook and the telephone line is idled.
There exist other systems, some of which are utilized in mobile radio communications, in which the public switched telephone network is utilized to provide both data and audio communications between plural mobile radio cell switching systems and a centralized hub controller. In this type of environment, the communications are of the dispatch type, or better known as push-to-talk (PTT). Each time a mobile radio is keyed, an idle radio channel is selected. The originating party may typically speak for several seconds, in which event the audio signals are transmitted via the mobile radio antenna to a local cell switching system. The cell switching system locally associated with the originating communication then transmits via an antenna the dispatch radio signals to other mobile radios in the radio transmission area. The originating cell switching system also transmits data signals on a data line, and after a talking path is established, transmits the audio signals via a telephone line through the PSTN to the hub controller. Based on the group to which the originating mobile radio is assigned, the hub controller bridges the telephone line from the originating cell switching system to the corresponding telephone lines of yet other cell switching systems so that all the mobile radios assigned to the group can receive the audio signals. In this manner, the destination cell switching systems that are located outside the radio transmission range of the transmitting cell can nevertheless receive the audio portion of the dispatch communications.
Even though the communications between each of the various mobile radios may be only a few seconds, the telephone audio and data lines interconnecting the cell switches to the hub controller are always "off-hook", thereby remaining ready to carry any of the dispatch audio signals. It can be appreciated that if a telephone line had to be dialed and established for each communication, the delay period would be unacceptable and the bidirectional communications of audio signals would be extremely cumbersome and burdensome. It can be seen that an expedited completion of a dispatch-type communication is of utmost importance.
Many mobile radio systems are configured so that a respective data line, which is typically a telephone line carrying modem signals, is coupled between each cell switch and the hub controller. Associated with each data line are one or more telephone lines for carrying the audio dispatch communications noted above. Depending upon the configuration of the system, the telephone lines can be idled, if not used within a period of several minutes. In such a system, during periods of low usage, at least one audio line of a cell switch will always remain active, and the remaining audio lines may be automatically disabled after a predefined time, but reactivated should the dispatch traffic increase to a level such that one audio line cannot handle the traffic conditions without encountering significant delays.
It is a well known practice to process dispatch communications of different groups of mobile radios by a single cell switching system. One group of mobile radios providing communications between police officers, and another group which is allocated for communications between taxi drivers can all be accommodated by a single cell switching system. The cell switching system maintains the communications separated between the groups by the use of respective group identification numbers (GID) that are uniquely associated with each group. Each mobile radio of a group has programmed therein the unique group identification so that when the mobile radio is keyed, the group identification is transmitted to the cell switching system in a frame of data. Accordingly, the cell switching system can then retransmit data with the GID embedded therein so that the other mobile radios in the group can decode the same and receive the wireless transmission. Other mobile radios of a different group not programmed with the unique GID cannot receive the communications, and thus the dispatch-type of communications can remain distinct as between the various groups.
It is possible for each cell switching system to accommodate many different groups, ranging up to 255 groups. In addition, any number of mobile radios can be active within a group. It can be appreciated that with the number of mobile radios being active, and with the communications being generally one or two seconds long, it is necessary for the cell switching system to quickly process the incoming key command and provide communications to the other members of the group. The processing must be efficient and fast. In practice, geographically remote cell switching systems are interconnected by a hub controller. Standard telephone lines coupled between the hub controller and the various cell switching systems provide the transmission medium in which dispatch communications from a group in one cell can be coupled to the members of the same group located in another cell. In this situation, the hub controller must not only determine which group is to be involved in a dispatch communication, but also must determine which destination cell sites are then serving the various members of the group. This extended range of service tends to slow down the processing of the dispatch calls, in that the hub controller must not only determine which cell sites might be involved as a destination for the dispatch call, but also the particular telephone lines connected thereto must be polled to determine if they are idle. Moreover, the key command from the originating mobile radio must be retransmitted by the hub controller over respective data lines connected to the destination cell switching systems.
The utilization of a hub controller and multiple cell switching systems to provide wide-area coverage for groups of mobile radios is further exacerbated when two mobile radios belonging to the same group attempt the initiation of dispatch calls at the same time. This contention for service must be resolved, as it is impractical for destination mobile radios of the group to simultaneously receive different originating dispatch communications. Again, if such a contention for service is not resolved in an expedited manner, the entire dispatch communication system can be bogged down with the processing of the calls by the hub controller.
From the foregoing, it can be seen that a need exists for a technique to manage dispatch communications in an efficient manner to thereby improve the throughput of the system. Another need exists for an improved technique for determining the various members of a mobile radio group, and with which cell switching system they are presently associated so that the voice message from the originating mobile radio can be efficiently broadcast to the appropriate cell switching systems. Another need exists for an efficient manner of maintaining an account of the switching connections associated with dispatch calls so that when a communication is terminated, the apparatus of the switching path can be quickly idled and prepared for reuse. Yet another need exists for an efficient technique to resolve the attempted simultaneously initiation of a dispatch communication by two members of the same group.